The ABC's of a Bone Density Test
September 16, 2002 04:05:55 PM PST, HealthScout News
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By Kathleen Doheny
HealthScoutNews Reporter
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MONDAY, Sept. 16 (HealthScoutNews) -- As medical tests go, a bone mineral density test or BMD, is one of the simplest.

There's no pre-test fasting or other preparation. No poking or prodding or needles involved. The technician records your weight and height, and might ask you to remove clothing with zippers or metal parts, such as your bra.

Then, you simply lie flat and still on a table during the "gold standard" bone density test, called a DEXA (dualenergy x-ray absorptiometry) while a machine that uses small amounts of radiation passes overhead.

"The test tells you how much bone mineral, the stuff that absorbs the X-ray, is there," says Dr. Robert Heaney, professor of medicine at Creighton University and a scientist at its Osteoporosis Research Center. And it can give you a good idea of your risk for osteoporosis, the "fragile bone" disease that can result in painful, costly fractures of the hip, spine and other areas.

It's all over in 10 minutes or less.

Then comes the more difficult part: interpreting the results and figuring out what to do next.

Your results are compared with those of healthy young women of your ethnicity at their peak bone mass, at age 30. And you are also compared to women of your ethnicity in your age bracket. The results may be expressed in a variety of ways: as a percentile, as a percent or as a standard deviation.

Ask for your results to be given to you in standard deviation form, experts say.

"The standard deviation result may be the most useful," says Lynn Chard-Petrinjak, spokeswoman for the National Osteoporosis Foundation. And probably the easiest for consumers to interpret.

The World Health Organization has established categories based on standard deviations of what is normal and what is not for bone mineral density, she says, and the NOF and others use these categories. In medical literature, when treatment is discussed, it is described for various standard deviations, she adds.

The result you should pay the closest attention to is how your bones compare to those of a healthy young woman of your ethnicity, says Dr. J. Michael Uszler, a nuclear medicine physician on staff at Santa Monica-UCLA Medical Center and an expert in bone mineral density testing.

"Forget about age-matched controls," he says, because many of them, especially if they are 50-plus, already have reduced bone mass density. Your fracture risk is more relative when you compare the quality of your bones to those of younger women of your own ethnicity, he says.

The result compared to younger women is often called the T-score; the result compared to women your own age is the Z-score.

So what's normal and what's not so good? According to the WHO categories, widely accepted by other experts, a normal bone density is within one standard deviation (SD) of the young adult mean, which means plus 1 or minus 1 SD.

Low bone mass is defined as 1 to 2.5 SD below the young adult, which means an SD of minus 1 to minus 2.5.

Bone density that is 2.5 SD or more below the young adult mean is defined as osteoporosis. So a result of minus 2.5 SD or greater is osteoporosis.

What you do next depends on the results and your doctor's judgment. If osteoporosis is present, you may be prescribed several of the bone-building drugs such as alendronate, raloxifene or risedronate. Hormone replacement therapy also builds bone.

If you pass the test without evidence of bone mass loss, your doctor may recommend repeating the test in four or five years, Uszler says, depending on a variety of factors such as your age and other risk factors. If your bone mass is not as good as it should be but you have no evidence of osteoporosis, he may suggest a repeat test in two years or so.

Requests for DEXA tests, which cost about $125 to $300, have increased at some centers in the wake of the release of the Women's Health Initiative study, which found that using hormone replacement therapy more than 5.2 years increases the risk of breast cancer, heart attack and stroke but protects against fractures.

While HRT builds bone, some women who decided to go off hormones are now wondering about their bone mineral density status, and asking for bone density tests. One California center says requests are up by about 20 percent since the WHI study results were released.

What To Do

For more information on bone density testing, see the National Osteoporosis Foundation, which also has information on bone-building treatments.


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